Lamp combination comprising an improved envelope for tungsten-halogen incandescible projection lamp

ABSTRACT

An incandescible projection lamp combination which utilizes a vitreous borosilicate glass, light transmitting envelope. The lamp utilizes a tungsten filament and a bromine containing atmosphere. The power loading is about 13.6 to 16.4 watts per cubic centimeter of lamp volume, whereby light output is essentially constant throughout life.

United States Patent DeCaro et al. 51 Mar. 7, 1972 [54] LAMP COMBINATION COMPRISING [56] References Cited AN IMPROVED ENVELOPE FOR TUNGSTEN HALOGEN UNITED STATES PATENTS [NCANDESCIBLE PROJECTION LAMP 3,555,338 1/1971 Scoledge et al. ..3l3/l 15 X 3,209,188 9/1965 Freeman ...313/222 X [72] In nt s Arisfide Edison; Nicholas J- 3,431,448 3/1969 English ..3 13/223 X Rainone, Clifton, both of NJ. Primary ExaminerRaymond F. Hossfeld [73] Asslgnee' wesunghouse Electric Corporation Attorney-A. T. Stratton, W. D. Palmer and Walter Sutcliff sburgh, Pa.

[22] Filed: Apr. 15, 1970 [57] ABSTRACT [2]] Appl. No.: 28,663 An incandescible projection lamp combination which utilizes a vitreous borosilicate glass, light transmitting envelope. The lamp utilizes a tungsten filament and a bromine containing at- [52] US. Cl ..3l3/22l, 313/174, 313/222, mosphere The power loading is about to 164 watts per 313/223 cubic centimeter of lamp volume, whereby light output is es [51] Int. Cl. ..H0lk l/50 Sentiany constant throughout life [58] Field of Search..... ....313/l15, 221, 222, 223,174

8 Claims, 1 Drawing Figure Patented March 7, 1972 3,648,094

l4 IJ1 .1 I 4 Mam-n3 ll l H U WITNESSES A 7/ 1 V us I e e Gro 1 1 and Nicholas J. Roinone ATTORNEY LAMP COMBINATION COMPRISING AN IMPROVED ENVELOPE FOR TUNGSTEN-HALOGEN INCANDESCIBLE PROJECTION LAMP BACKGROUND OF THE INVENTION The tungsten-halogen incandescible lamp is now well known in the art. This lamp prevents blackening of the transmissive envelope by a process, which although not completely understood, involves the formation of relatively volatile tungsten halides. The conventional tungsten-halogen lamp has up to this time employed quartz, or very high-silica content material, at least about 99 percent silica, content glass which is highly temperature resistant, as the light transmissive envelope. The quartzenvelope is closely spaced from the filament, and the power input loading is generally greater than about 70 watts per cubic centimeter of lamp volume, to provide a relatively high-cool spot temperature of approximately 500C.

The use of quartz or other such high-silica content glass has made such devices expensive because of material costs and also because of manufacturing techniques required for working this high-temperature resistant material. The quartz envelope must be handled to avoid fingering contamination, which can lead to devitrification and early lamp failure. The quartz envelope also requires an expensive ceramic base seal which is not as reliable as the standard metal to hard glass seal.

The tungsten-halogen lamp is used as a standard long lifetime lighting source and also as a relatively short lifetime, high-efficiency light-generating projection lamp. A low-voltage, high-power input per cubic centimeter of bulb volume, lamp utilizing a hard glass, Pyrex envelope is disclosed in Canadian Pat. No. 763,062 issued July 11, 1967. The power inputs described in this teaching were from about 30 watts to 95 watts per cubic centimeter of lamp volume.

SUMMARY OF THE INVENTION It has been discovered that an improved projection lamp can be made having a bromine containing operating atmosphere, and an envelope of vitreous borosilicate glass which for example has a softening temperature of about 820 C. or slightly less. The lamp design is such that the power input is from about 13.6 to 16.4 watts per cubic centimeter of lamp volume. The lamp envelope cool spot temperature is to be maintained at a value sufficient to insure operation of the bromine-tungsten cycle. This lamp exhibits a very high-maintenance value of light emission during the operating lifetime.

DESCRIPTION OF THE DRAWINGS The sole figure is an elevational view of a representative embodiment of the projection lamp of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The lamp comprises a light-transmissive envelope 12, and is formed of a vitreous light transmissive material which can withstand the effective operating temperature and effects of the bromine containing atmosphere. This vitreous material is a borosilicate glass, for example Pyrex No. 7740 glass, a product of Coming Glass, which typically has an approximate composition of 80 percent silica, l4percent boron oxide, 4percent soda (N820), and 2percent alumina. This vitreous material exhibits a high-chemical stability, a low coefficient of thermal expansion, and a high-heat shock resistance, all of which properties are important in the present instance because of the reactivity of the halide atmosphere, the highoperating temperatures of projection lamps and the resultant shock upon cycling the lamp on and off. The softening temperature of the particular borosilicate glass which is preferred is about 820 C. Of course, other borosilicate glasses exhibiting good thermal stress resistance can be utilized.

The conventional metallic base 14, for example of aluminum, encompasses the sealed base of the envelope. A plurality of conductive lead-ins 16, of for example tungsten, are

hermetically sealed through the lamp envelope 12 via conventional hard glass to metal seals. The inwardly extending portions of the lead-ins 16 are partially supported by the bridges 18, which are formed of high-temperature resistant, vitreous, light-transmissive material such as quartz. The incandescible tungsten filament 20 by way of example comprises an eight coil biplanar filament, with four coils in each plane. The ends 22 of the filament 20 are electrically connected to the lead-ins 16 by winding slip coils 24 over the lead-ins with the filament ends between the slip coil and the lead-ins. Coil support members 26 whichare embedded in the bridges l8, serve to retain the filament in position. This lamp also generally has an internally disposed fuse, not shown, which is connected in series with a conductive lead-in.

A nominal 500 watt lamp which is designed to operate at from to volts has the following approximate dimensions. The lamp bulb is a standard size, what is termed a T-l0 bulb, which has a cylindrical body portion and a domed top portion, a diameter of about 32 mm., a length of about 60 mm. from the top of the bulb to the base seals, and the lamp volume is approximately 33 cc. The filament coils are formed of 7.35 mil (0.187 mm.) diameter tungsten wire wound at 93.2 turns per inch, with the overall length of each coil being about 6.35

The lamp is filled with nitrogen to a pressure of from 950-1050 torrs, and with methylene bromide in an amount (when vaporized) of about 0.4 percent by volume of the fill. The lamp can be preheated during manufacture to decompose the methylene bromide, or is preferably operated for a short period to decompose the methylene bromide and provide a bromine partial pressure in the lamp. The residue from the decomposed methylene bromide will condense at the base area of the envelope, and generally at that portion of the envelope which is obscured from view by the metallic base which surrounds the bottom of the envelope.

The particular T-l0 size lamp parameters described result in an operating cool spot of about 340 C., which is observed at the envelope-base shell junction. This temperature is high enough to insure vaporization of the bromine constituent which prevents envelope blackening, yet is not so high as to cause failure of the envelope. The lamp envelope is subjected to a range of operating temperatures for the T-l0 bulb embodiment described, for example about 425 C. at the bulb top and a peak of about 525 C. adjacent the filament area.

It has been observed that during the lifetime of this particular projection lamp that substantially no diminution of lumen output occurs. The lifetime of this particular lamp is the standard lifetime for the particular coil size utilized. The tungstenbromine cycle, by which the vaporized tungsten from the filament is kept from depositing on the bulb, wall prevents the localized destructive heating of the bulb which occurs in a nonhalogen lamp as the dark tungsten deposits absorb further infrared radiation.

Nitrogen is specified as the fill gas in the preferred embodiment to diminish the possibility of arcing between adjacent portions of the filament in this embodiment. If sufficient spacing is provided between the coils, other insert gases can be utilized as the fill gas.

In summary, the lamp of the present invention ofiers ad vantages over both a standard nonhalogen containing incandescent lamp and a quartz-envelope, halogen containing lamp. The present lamp shows greatly increased lifetime and maintenance during life as compared to the nonhalogen containing lamp. The present lamps can offer the same light characteristics as the quartz envelope halogen lamp and some improvement in life, at a substantially lower cost. The use of a hard glass envelope instead of quartz simplifies the basing requirements and avoids the dangers of devitrification resulting from handling the quartz. The lamp of the present invention is particularly adapted for motion picture projection systems which have a controlled environment with forced air ventilation; however, the lamp can be operated in air without bulging.

A particular advantage of the hard glass T-l bromine lamp embodiment over a comparable wattage and light output producing T-6 quartz-bromine lamp is that the lamp of the present invention can produce a higher average comer to center screen lumen percentage ratio in a given projector system. This improvement is apparently caused by the diffusing effect of the bromine and nitrogen occupying a larger volume which more uniformly fills the optical system. For example, for a 31.5 inch by 45.5-inch screen, using the same projector system, the prior art T-6 quartz bromine lamp produced an average corner to center screen lumen percentage ratio of 66 while the T-lO hard glass bromine lamp yielded a ratio of 83.

What we claim is:

l. A projection lamp combination having improved life and high brightness, as well as improved initial average screen lumens, and improved average corner to center screen brightness ratio when used in a projection system, said projection lamp comprising:

a. a sealed light-transmissive envelope formed of borosilicate glass enclosing a volume which is predetermined with respect to the power input at which said lamp is designed to operate;

b. an incandescible tungsten filament supported and positioned within said envelope in a generally central location to provide relatively uniform heating of said envelope during lamp operation, said filament adapted to be operated at a predetermined wattage input, and electrically connected to lead-in conductors which are sealed through said envelope;

c. an inert gas atmosphere at a predetermined pressure, and a predetermined amount of bromine enclosed within said d. The volume of said lamp being determined with respect to the power input such that the ratio of nominal wattage input to lamp volume is from about 13.6 to 16.4 watts per cubic centimeter, whereby a bromine-tungsten cycle is maintained so that said light output is essentially constant throughout the design life of said lamp and the integrity of said envelope is maintained throughout the design life of said lamp.

2. The combination as specified in claim 1, wherein said borosilicate glass has a composition of about silica, 14% boron oxide, 4% soda, and 2% alumina.

3. The combination as specified in claim 1, wherein said bromine is supplied to said lamp as methylene bromide.

4. The combination as specified in claim I, wherein said inert fill gas is nitrogen at a pressure of from about 950 to 1,050 torrs.

5. The combination as specified in claim 3, wherein said methylene bromide is supplied in an amount of 0.4% by volume of the operative atmosphere.

6. The combination as specified in claim 1, wherein said filament is a biplanar coiled filament.

7. The combination as specified in claim 1, wherein the minimum temperature of said envelope when said lamp is operated at nominal wattage input is about 340 C. and the maximum envelope temperature is about 525 C.

8. The combination as specified in claim 7, wherein said lamp has a nominal wattage input of 500 watts, said envelope has a cylindrical body portion with a diameter of about 32 mm., and said envelope encloses a volume of about 33 cc.

inn: 

2. The combination as specified in claim 1, wherein said borosilicate glass has a composition of about 80% silica, 14% boron oxide, 4% soda, and 2% alumina.
 3. The combination as specified in claim 1, wherein said bromine is supplied to said lamp as methylene bromide.
 4. The combination as specified in claim 1, wherein said inert fill gas is nitrogen at a pressure of from about 950 to 1,050 torrs.
 5. The combination as specified in claim 3, wherein said methylene bromide is supplied in an amount of 0.4% by volume of the operative atmosphere.
 6. The combination as specified in claim 1, wherein said filament iS a biplanar coiled filament.
 7. The combination as specified in claim 1, wherein the minimum temperature of said envelope when said lamp is operated at nominal wattage input is about 340* C. and the maximum envelope temperature is about 525* C.
 8. The combination as specified in claim 7, wherein said lamp has a nominal wattage input of 500 watts, said envelope has a cylindrical body portion with a diameter of about 32 mm., and said envelope encloses a volume of about 33 cc. 